操作数组的常见的方法有forEach、map、filter，其中map和filter方法的返回值都是数组，forEach的返回值是undefined，可以理解为没有返回值。
由于原生的数组对象中，没有concatAll方法，所以打算自己实现一个。concatAll方法要做的事情很简单，就是把一个二维数组转成一维。

Array.prototype.concatAll = function() {
    var result = [];

    // 用 apply 完成
    this.forEach((array) => {
        result.push.apply(result, array);
    });

    return result;
};

这里我们用到了apply方法。apply方法会调用一个函数，apply方法的第一个参数会作为被调用函数的this值，apply方法的第二个参数（一个数组，或类数组的对象）会作为被调用对象的arguments值，也就是说该数组的各个元素将会依次成为被调用函数的各个参数。我们利用apply方法的这种特性，实现了数组的解构（降维）。
接下来，我们来解决一个问题：

var courseLists = [{
  "name": "My Courses",
  "courses": [{
    "id": 511019,
    "title": "React for Beginners",
    "covers": [{
      width: 150,
      height: 200,
      url: "http://placeimg.com/150/200/tech"
    }, {
      width: 200,
      height: 200,
      url: "http://placeimg.com/200/200/tech"
    }, {
      width: 300,
      height: 200,
      url: "http://placeimg.com/300/200/tech"
    }],
    "tags": [{
      id: 1,
      name: "JavaScript"
    }],
    "rating": 5
  }, {
    "id": 511020,
    "title": "Front-End automat workflow",
    "covers": [{
      width: 150,
      height: 200,
      url: "http://placeimg.com/150/200/arch"
    }, {
      width: 200,
      height: 200,
      url: "http://placeimg.com/200/200/arch"
    }, {
      width: 300,
      height: 200,
      url: "http://placeimg.com/300/200/arch"
    }],
    "tags": [{
      "id": 2,
      "name": "gulp"
    }, {
      "id": 3,
      "name": "webpack"
    }],
    "rating": 5
  }]
}, {
  "name": "New Release",
  "courses": [{
    "id": 511022,
    "title": "Vue2 for Beginners",
    "covers": [{
      width: 150,
      height: 200,
      url: "http://placeimg.com/150/200/nature"
    }, {
      width: 200,
      height: 200,
      url: "http://placeimg.com/200/200/nature"
    }, {
      width: 300,
      height: 200,
      url: "http://placeimg.com/300/200/nature"
    }],
    "tags": [{
      id: 1,
      name: "JavaScript"
    }],
    "rating": 5
  }, {
    "id": 511023,
    "title": "Angular2 for Beginners",
    "covers": [{
      width: 150,
      height: 200,
      url: "http://placeimg.com/150/200/people"
    }, {
      width: 200,
      height: 200,
      url: "http://placeimg.com/200/200/people"
    }, {
      width: 300,
      height: 200,
      url: "http://placeimg.com/300/200/people"
    }],
    "tags": [{
      id: 1,
      name: "JavaScript"
    }],
    "rating": 5
  }]
}];

/* 
var result = courseList
不得直接使用索引 covers[0]，请用 concatAll, map, filter, forEach 完成
result 结果为 [
    {
      id: 511019,
      title: "React for Beginners",
      cover: "http://placeimg.com/150/200/tech"
    }, {
      id: 511020,
      title: "Front-End automat workflow",
      cover: "http://placeimg.com/150/200/arch"
    }, {
      id: 511022,
      title: "Vue2 for Beginners",
      cover: "http://placeimg.com/150/200/nature"
    }, {
      id: 511023,
      title: "Angular2 for Beginners",
      cover: "http://placeimg.com/150/200/people"
    },
 ]
*/

这种类似的问题在我们实际的开发中是经常用到的，本人的答案如下：

var result = [];
var rel = courseLists.map((courseList) => {
    return courseList.courses;
}).concatAll().forEach((courseListItem) => {
    return result.push({
        id: courseListItem.id,
        title: courseListItem.title,
        cover: (courseListItem.covers.filter((coversItem, idx) => {
            if (idx === 0) {
                return coversItem;
            }
        }))[0].url
    })
});
console.log(result);